In a bed, the mattress cover and the bedsheet are usually edgewisely (releasably) secured to the mattress by their engagement under the peripheral underface of the mattress, wherein the weight of the mattress will bias the sheets in their extended position. It is desirable that the two sheets should continue to be fully extended during the whole night instead of entangling in a bulge, for a variety of reasons including proper thermal insulation, comfort and preventing wear of the mattress proper.
However, due to non-voluntary movements of the sleeping person in his bed associated with dream activity, the sheets may become accidentally dislodged from the underface of the mattress. This problem is more acute with waterbeds, in that the waterbag has a flexible wall wherein the securing action of the edgewise biasing force thereof against the edge section of the sheets is reduced.
A search of prior art has unveiled the following: Canadian patent #143,248 dated October 1912; Canadian patent #481,762 issued March 1952; U.S. Pat. Nos. 2,930,053 issued March 1960; and 3,092,848 issued June 1963. These patents show various strap means for securing the sheets of a bed about the mattress. The Canadian patent 481,762 in the name of Block, Sher and Block, is particularly interesting in that it discloses strap means 16, 22 positioned under the mattress of a bed for releasably securing a bedsheet 30 and a mattress cover 10. A general problem of these sheet securing means is that they usually require for their disengagement that the mattress be lifted from the box mattress. This problem is compounded in the case of waterbeds, wherein the weight of the body of water contained in the waterbag, which may be several tens of kilograms, may discourage use of such bedsheets securing means.
Moreover, waterbeds require special bedsheets having corner pockets, which special bedsheets can easily cost more than twice the price of conventional bedsheets.